928aa nuclear phosphoprotein; 110 kDa; pRB is phosphorylated by members of the cyclin-dependent kinase (cdk) system prior to the entry into S-phase; hypophosphorylated pRB binds to members of the E2F family of transcription factors; binding to E2F is mediated by by two domains within pRB (pocket domains)

Expression

ubiquitous

Localisation

nucleus

Function

cell cycle regulation, differentiation

Homology

pRB, p107, and p130 constitute a small family of nuclear proteins with significant sequence similarity in two discontinuous areas (pockets domains); conditional on the phosphorylation status, these pocket proteins can bind transforming proteins of DNA tumor viruses as well as nuclear proteins

Mutations

Germinal

germline mutations in the RB1 gene are causative for hereditary predisposition to retinoblastoma; the spectrum of predisposing mutations includes large deletions (about 20%), single base substitutions (about 50%) and small length mutations (about 30%); most mutations are associated with almost complete penetrance: some rare alleles show incomplete penetrace and reduced expressivity (low penetrance retinoblastoma)

Somatic

in retinoblastomas, both RB1 alleles are mutated; in addtion to the mutational spectrum of germinal mutations, retinoblastomas can show loss of heterozygosity and hypermethylation at the CpG-island associated with the 5'-end of the RB1 gene

RB1 gene mutations have been observed in several tumor entities. In lung cancer there is a notable high frequency of RB1 gene mutations although no increased incidence of these tumors has been observed in carriers of a germinal RB1 gene mutation.

Bibliography

The interaction of RB with E2F coincides with an inhibition of the transcriptional activity of E2F.